1. Field of the Invention
The present invention relates to a measuring method and a measuring apparatus for measuring optical energy absorption ratio of a processing object substrate to a plain substrate on which no pattern is formed, as well as a thermal processing apparatus employing such a measuring technique.
2. Description of the Background Art
In an ion activation step of a semiconductor wafer after being subjected to ion implantation, there has heretofore been used a thermal processing apparatus such as a lamp annealing apparatus using halogen lamps. In such a thermal processing apparatus, the ion activation of a semiconductor wafer is carried out by heating (annealing) the semiconductor wafer to temperatures of, for example, approximately 1000° C. to 1100° C. This thermal processing apparatus is constructed so as to elevate the temperature of the substrate at a speed of about several hundreds of degrees per second, by utilizing the energy of light irradiated from the halogen lamps.
However, even when the ion activation of a semiconductor wafer is executed with a thermal processing apparatus that elevates the temperature of the semiconductor wafer by halogen lamps at a speed of about several hundreds of degrees per second, the profile of ions implanted into the semiconductor wafer becomes round. That is, it has been found to cause the phenomenon that ions diffuse by heat. In case that this phenomenon occurs, even if ions are implanted at a high concentration into the semiconductor wafer surface, the implanted ions may diffuse. This introduces the problem that it is necessary to implant more ions than necessary.
In order to solve the above problem of ion diffusion, for example, Japanese Patent Application Laid-Open Nos. 59-169125 and 63-166219 have proposed such a technique that only the temperature of the surface of a semiconductor wafer after being subjected to ion implantation is elevated in an extremely short period of time (not exceeding several milliseconds) by irradiating flashlight to the surface of the semiconductor wafer by use of xenon flash lamps, etc. For the temperature elevation in a very short time by the xenon flash lamps, the ions will not have a sufficient time to diffuse. Therefore only the ion activation is executable without rounding the profile of ions implanted into the wafer.
However, in a thermal processing apparatus using xenon flash lamps, very enormous optical energy is irradiated to the wafer surface in an extremely short period of time. This causes a rapid elevation of the surface temperature thereof, so that only the wafer surface expands rapidly. It has been found that any excess energy irradiated from the xenon flash lamps causes only the wafer surface to expand rapidly, thus leading to a slip in the wafer surface, and the wafer break in the very worst case. On the other hand, it is impossible to carry out ion activation if irradiation energy is insufficient. It is therefore important to optimize the range of optical energy irradiated from the xenon flash lamps.
In general, with flash lamps of which irradiation time is extremely short, it is impossible to perform feedback control of lamp output on the basis of the measuring result of the temperature of a semiconductor wafer. Hence, there is first performed ion implantation of a plain bare wafer on which no pattern is formed, and the plain wafer after being subjected to the ion implantation is then subjected to actual light irradiation. Thereafter, the characteristics after processing (e.g., sheet resistance value) is measured, and the optical energy irradiated from the xenon flash lamps is adjusted on the basis of the measuring result.
However, a pattern is already formed on a wafer to be processed practically, and therefore it often has optical absorption characteristics different from that of a plain wafer. Usually, even if the light of the same energy is irradiated, a wafer on which a pattern is formed tends to absorb more optical energy than a plain wafer. Hence, even if the irradiation energy to the plain wafer is optimized, the wafer processed practically absorbs even more, which can create the problem of causing wafer break. To avoid this, optimum irradiation energy for plain wafer must be compensatory to each processing object wafer.